I would think it was this one. I have too many types from them and not sure which is which - but this looks right:
You could put your own 'neck' to support it. I never even thought about that.... but I would just use some wood. Probably a plywood (3mm or 4mm) disc, cut in half to allow it to be joined. Or maybe even a pair of those from 2.5mm, and cross them at 90deg to be a stronger end result. Glue them onto the motor extension and they can just wedge (slightly) into the housing hole.
Or maybe even more elaborate.... 19mm pine, machined (saw, file, drill) to form the same idea but being 19mm long for more support. Sort of ending up like KnifeLiddle's way of mounting a bigger outrunner.